Various studies have been given to high strength aluminum alloys obtained from an alloy containing amorphous metal, a supersaturated solid solution, and microcrystalline metal which is obtained by rapid quenching. For example, JP-B-6-21326 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses that a rapid quenching and solidification of a ternary alloy represented by the formula Al.sub.a M.sub.b X.sub.C (wherein M represents at least one element selected from Cr, Mn, Fe, Co, Ni, Cu, Zr, Ti, Mg and Si; X represents at least one element selected from Y, La, Ce, Sm, Nd, Nb and Mm (mish metal); a, b, and c are atomic percentages, in which a is from 50 to 95, b is from 0.5 to 35 and c is from 0.5 to 25) yields an amorphous alloy or a composite of amorphous matter and microcrystalline matter, each having a tensile-strength of from 853 to 1010 MPa (from 87 to 103 kgf/mm.sup.2) and a yield strength of from 804 to 941 MPa (from 82 to 96 kgf/mm.sup.2).
The resulting aluminum alloy has a high tensile strength which is twice or more that of conventional crystalline aluminum alloys, but its Charpy impact strength is less than about one fifth of that of conventional ingot aluminum.
JP-A-5-1346 (the term "JP-A" as used herein means an "unexamined published Japanese patent application) discloses that an aluminum alloy having a tensile strength of from 875 to 945 MPa (from 89.2 to 96.3 kgf/mm.sup.2) and an elongation in tensile test of from 1.7 to 2.9% is obtained by rapid quenching and solidifying an alloy system represented by the formula Al.sub.a M.sub.b Ln.sub.c or Al.sub.a M.sub.b X.sub.d Ln.sub.c (wherein M is at least one element selected from Co, Ni and Cu; Ln is at least one element selected from Y, rare earth elements and Mm; and X is at least one element selected from V, Mn, Fe, Mo, Ti and Zr). The metallographic structure of the alloy has an average grain size of from 0.1 to 80 .mu.m. The matrix is aluminum or a supersaturated solid solution of aluminum, and fine particles of an intermetallic compound in a stable or metastable phase having a particle size of 10 to 500 nm are distributed in the matrix. The term "matrix" as used in the present invention means the host phase which encloses the other phase therewith.
In the case of the alloy disclosed in JP-A-5-1346-in which fine intermetallic compound particles at- the order of nanometers are dispersed in the supersaturated solid solution matrix, the finely dispersed intermetallic compound particles expand upon application of heat. Therefore, the toughness of the aluminum alloy is considerably reduced at a certain temperature or higher.
Therefore, the aluminum alloys described in JP-B-5-21326 and JP-A-5-1346 are both unsuitable for use as a material for machine parts and automotive parts that are required to have high reliability.
In order to overcome the above problems, the present inventors have studied the microstructures of aluminum alloys in the order of nanometers and their mechanical characteristics. They have found that, when a conventional supersaturated solid solution is heat-treated, there is produced a clear crystalline grain boundary between a precipitated intermetallic compound and the Al matrix, and the anchoring of dislocation upon plastic deformation concentrates at the grain boundary. This interferes the attempt to increase the toughness.
The inventors considered that concentration of dislocation anchoring might be prevented by using a modulated structure (a microstructure having regular fluctuations in concentration) having no clear boundaries between an intermetallic compound and an Al matrix. It was revealed that such a modulated structure exhibits high toughness while the intermetallic compound is precipitating, but the toughness is considerably reduced with the progress of precipitation till complete precipitation. This is because clear crystalline grain boundaries are formed between the Al matrix and the precipitate at the completion of precipitation, and dislocations upon plastic deformation are concentrated at the grain boundaries.